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It hasn’t rained in weeks. It is hot and your soil is getting really dry. So you turn on your sprinklers for a half hour. Do you have any idea how far down it has wet your soil? Do you how many inches of rain it was equivalent to? Did it get down into the root zone? Was the soil so dry it actually repelled the water?

Soil can be so dry it actually becomes hydophobic, and refuses to absorb water.

Soil can be so dry it actually becomes hydophobic, and refuses to absorb water.

So, you think you know your soil? Maybe you really don’t. Like people, it can keep secrets. And since understanding your soil is probably the single most important aspect to growing a lush healthy garden, it’s time to dig in and uncover what may be its biggest subterranean secret – how much should you water?

The root zones of various plants

The root zones of various plants

Here are some very broad “rules of thumb.” The effective root zone for most plants extends down to about 3 feet. An inch of rain will wet the soil to a depth of 1 foot, if there is no runoff and the soil is a sandy loam. If your soil trends to more sand it will penetrate further, and it will be more easily absorbed, but it won’t be retained as long. If your soil is more clay, it will not penetrate as deeply, or as readily, but will stay wet longer. So, a one-inch rain event on a clay hillside may only wet the soil to a depth of an inch. Whereas the same event on a level sandy soil may penetrate as much as 2 feet.

Click photo for an excellent article on how to evaluate your soil type.

If you use traditional above ground sprinklers, you  can get a decent idea of their volume per hour by placing a large level saucer on the ground and measure how much water it collects when you turn them on. Because most of the water from sprinklers is coming to the ground at an angle, your typical rain gauge will not be useful. a broad, low-walled, container will be best.

After you start monitoring the amount of rain you get, and the rate your sprinklers irrigate, you can then begin to understand how and why your soil may be keeping secrets from you. Each time you dig a hole to plant something, pay careful attention to whether there is any moisture down a foot or more. Most gardeners, for obvious reasons, manage to keep their soil wet for the first few inches at the surface. But in order for larger plants to get established and grow well, they need water to periodically percolate down much farther, into the primary root zone – where the secrets are kept.

You just turned off the sprinklers, but 12 inches down it is dry.

You just turned off the sprinklers, but 12 inches down it is dry.

It is a mistake we all make when the weather is dry. We try and save money. We see the weeds and grass growing well enough. But, if we dig a hole, we can see that past a few inches deep the soil is bone dry. And I emphasize bone dry – zero water is available where the majority of your big landscaping plant’s roots are. Do it, you will be surprised.

Getting water deep into your soil encourages the deep root growth that allows the big mainstays of your landscape (the trees and large bushes) to survive dry spells, high winds, and stay healthy in order to combat insects and diseases. Be aware that while your lawn, weeds, and flowers may look watered, your major landscaping plants may have dry roots.

And until you know and understand your soil, there is only one way to know. So, go dig a hole. If it has been dry for a while, I will bet that despite your irrigation the soil where your larger landscaping plant’s roots are will be dry.

rootzonesAnd examine each hole you dig when planting, to evaluate the different conditions in the different ones of your garden. And as you form a relationship with your soil, there are many tricks you can take advantage of. If you are on a hillside, you can build berms and use terraces. Direct water from where it runs off in heavy rains to where it can be retained and sink in. Use water that runs off your roof or hardscape and direct it to where it will be better used. When the soil gets saturated to a depth of 2-3 feet, the larger plants in your garden can go for weeks without water and be much happier.

Drip irrigation is great for getting water deep. So is using and filling basins around your larger plants. But it is much harder to correctly know what is going on underground. The only way is to look. But before long, you will know without looking. So, if you want a lush green garden, get to know your soil and don’t let it keep secrets.

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